The high speed assembly lines found in modern manufacturing facilities are typically equipped with assembly machines designed to run down and tighten automatically a plurality of threaded fasteners to a predetermined torque value. These machines, known as multiple spindle torque units, are essential components of engine assembly lines which must operate at a moderate to high rate of speed in applying and tightly securing large numbers of threaded fasteners. The amount of tightness or torque placed on each fastener is controlled by an electronic device known as a torque transducer which is an integral component of each spindle.
A torque measuring transducer, such as a spindle mounted strain gauge, tends over time to give false readings due to drift in the measuring circuit to which the transducer is attached. Inaccurate readings may also result from deterioration of the bonding between the transducer and its support and/or non-resilient deformation of the spindle due to stress cycling and the like. Periodic checking and calibration is, therefore, a prerequisite for reliable transducer operation. Each spindle must be checked from time to time to test and verify that the correct amount of torque is being applied to each threaded fastener. Since the spindle torque measurements are used to control the machine tool and to check proper operation, periodic calibration of the torque measuring transducer mounted on the spindle of the machine tool is vitally important to insure that the machine tool will function properly and with precision.
Calibration of a spindle mounted torque transducer typically involves the steps of applying a known torque to the spindle and adjusting the measuring circuitry and/or transducer to produce an output reading corresponding to the known torque. Obviously, the accuracy of this method of calibration will depend, in large part, on the degree to which the applied torque can be accurately determined and controlled.
The apparatus and methods currently available for the testing and verification of the calibration accuracy of the torque transducers associated with each spindle of a multiple spindle torque unit, while effective for the desired purpose, are nonetheless, subject to limitations. One way to assure that a known torque is accurately applied to each spindle is to test each spindle at a bench testing unit remote from the machine tool in which the spindle is normally mounted. This involves at least partially disassembling the machine tool so that each spindle can be removed for testing and then reinstalling each spindle following calibration. Although very accurate calibration of the spindle transducers is possible with this method, the likelihood of transducer damage between calibration and spindle reinstallation which would disturb the calibration must be considered. Bumping or jarring the spindle could occur with sufficient force during reassembly of the machine tool to cause the torque transducer to be miscalibrated. Such an inaccuracy is not likely to be noticed even after the machine tool is put back in operation. When the inaccuracy is ultimately detected, the machine tool will have to be dismantled again and the transducer recalibrated and reinstalled. The bench testing method just described can also be quite costly. The machine tool is likely to be out of service for several days, requiring either that the assembly line be shut down or that an alternative torquing method be employed while the spindle transducers are being tested and calibrated.
A method and apparatus for calibrating a torque measuring transducer mounted on the spindle of a machine tool while the spindle remains mounted within the machine tool is disclosed in U.S. Pat. No. 4,253,325, assigned to the same assignee as the present invention. In the method disclosed therein, the spindle motor is pinned to restrict rotation during the calibration procedure so that the required circular motion is generated against the pinned motor and spindle transducer. A torque applying lever assembly is mounted between and supported by two adjacent spindles, on one of which is mounted the transducer to be tested. A first arm of the torque applying lever assembly is mounted by a torque coupling assembly to the spindle on which the transducer being calibrated is mounted, and a second arm in the form of a threaded shaft pivotally connected at one end to the first arm is threadedly mounted by a support coupling to an adjacent spindle. A master transducer is mounted with the torque coupling assembly on the spindle to be tested so that a known torque can be applied to the spindle by the torque applying lever assembly and the spindle transducer output compared with the master transducer output and appropriately calibrated. The apparatus and method disclosed in U.S. Pat. No. 4,253,325 has proved to be effective for the on line calibration of spindle transducers in machine tools which apply torques in the lower ranges. However, in machine tools operating above the 100 foot pound range, the system becomes distorted, with distortions of up to 20% being noted. Consequently, this system should be limited for use with smaller torque units that operate well below 100 foot pounds so that the accuracy of transducer calibration can be positively assured. In addition, this system must be mounted on two adjacent spindles for operation and, therefore, must be properly positioned on both spindles prior to the application of the known torque to the spindle to be tested to insure that the known torque will be accurately applied by the assembly and will be safely maintained for as long as required for calibration.
Other apparatus and methods for determining the torque output of a spindle or rotating shaft are disclosed in U.S. Pat. Nos. 3,115,029 to Better; 3,382,710 to Aubeges et al; 3,457,780 to Agostini and 3,797,305 to Haskell. While the devices disclosed in the aforementioned patents may function effectively to measure the torque of these various types of spindles or rotating shafts, none of these references discloses a means for applying a known torque, as measured by a master transducer, to calibrate a torque transducer mounted on the machine spindle.